Re: [eigen] Setting diagonal of a sparse matrix

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

I'm not sure if there is a better way to 'replace' the diagonal, if it 
does exist partially already. But setting the entire Matrix S to the 
diagonal of D goes like this:
  S = D.diagonal().asDiagonal();  // clears all off-diagonals, of course
If you know that there are no entries yet (or don't care), you can do 
the same with += or -= instead.
As the documentation says, S.diagonal()=... only works if all diagonal 
entries already exist.


Christoph

On 2016-09-22 15:14, Brad Bell wrote:
> How should one efficiently set the diagonal of an Eigen sparse matrix ?
>
> The following program does not compile if I set
>     USE_SPARSE_DIAGONAL 1
>
> # include <Eigen/SparseCore>
> # include <iostream>
>
> # define USE_SPARSE_DIAGONAL 0
> int main()
> {   using Eigen::Dynamic;
>     Eigen::Matrix<double, 3, 3> D = Eigen::Matrix<double, 3,
> 3>::Identity();
>     Eigen::SparseMatrix<double> S(3, 3);
>     D.diagonal() = D.diagonal() / 2.0;
> # if USE_SPARSE_DIAGONAL
>     S.diagonal() = D.diagonal();
> # else
>     for(size_t i = 0; i < 3; i++)
>         S.coeffRef(i, i) = D(i, i);
> # endif
>     std::cout << "S =\n" << S << "\n";
> }
> ~

--
 Dipl. Inf., Dipl. Math. Christoph Hertzberg

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